Previous efforts in the field of improving the removal of gaseous inclusions in glass melts have included the application of ultrasonics. The early work was done in small crucibles of molten glass or of a viscous medium, usually gylcerine. These efforts showed that ultrasonics probably could help to remove the gaseous inclusions. However, the encouraging results have never been pursued in a manner which is applicable to large glass-melting tanks as is indicated by the lack of follow-on studies in the open literature. Our own study indicates that the application of ultrasonics to large glass-melting tanks is both economically and technically attractive. If this new technology can be applied in a continuous operation, as much as 20 percent of the energy required to produce glass articles can be saved. A cursory economic analysis indicates that the successful application of this technology can save between $0.4 million and $9.8 million dollars per year for each glass tank to which it is applied.
To fully establish the proof of concept, two types of testing, scale modeling in viscous oils and crucible experiments in hot glass, are required. Scale modeling allows observation of the occurring phenomena in a medium that is at or slightly above room temperature. The crucible experiments in hot glass help to establish critical variables and material compatibility requirements for introducing this new technology into a continuous glass-melting tank. Hereinafter discussed are the design and results of scale modeling in a viscous oil medium, and a cursory economic analysis based on the results of the experiments. The results are also used to define the requirements for experimentation in crucibles of molten glass.
A typical method according to the present invention for removing gaseous inclusions from a highly viscous liquid containing both inclusions and dissolved gases comprises applying sonic energy in the liquid at an energy intensity sufficient to induce migration and coalescence of the inclusions in the liquid, and less than that required to produce substantial cavitation therein, so as to avoid substantial liberation of dissolved gases (which, as mentioned earlier herein, would take place during cavitation, forming bubbles and thereby increasing the quantity of inclusions needing to be removed); the sonic energy being applied until the volume density of the inclusions has been reduced to a desired level. Typically, the viscosity of the liquid is about 50 to 1000 poise, the energy intensity is about 0.003 to 15 watts per square centimeter, and the frequency of the sonic energy is about 16 Hz to 100 kHz.
The frequency of the sonic energy preferably is selected, and adjusted if necessary, to match the acoustic parameters of the liquid and the container holding it. The source of the sonic energy preferably is selected, and adjusted if necessary, to have an acoustic impedance that substantially matches the acoustic impedance of the liquid at the interface between the source and the liquid.
Typically, the frequency and the energy intensity are selected, and adjusted if necessary, to provide a mode of operation whereby the liquid is subjected to a cyclic component of stress that causes bubbles to collide and form larger bubbles, and to a substantially unidirectional component of stress, due to viscous losses and other mechanisms existing in an acoustic field (e.g. radiation pressure), that tends to drive the growing bubbles away from the source of the sonic energy and thus to facilitate their movement toward the surface of the liquid and the environs.
The invention is especially advantageous where the liquid is molten glass at a temperature of about 1200 to 1500 C., having a viscosity of about 50 to 1000 poise, where the intensity of the sonic energy is about 0.003 to 15 watts per square centimeter, and the frequency is about 16 Hz to 100 kHz. Where the liquid is molten glass, the frequency and the energy intensity typically are selected, and adjusted if necessary, responsive to the dynamic viscosity of the glass and the acoustic impedance at the interface between the source of the sonic energy and the glass, to provide a mode of operation wherein a substantial percentage of the bubbles in the glass migrate upward at rates at least about equal to a rate at which a bubble about 0.4 millimeter in diameter typically rises because of buoyancy through glass at a viscosity of about 100 poise.